1. Field of the Invention
The present invention relates to a silicon nitride ceramic sliding material excellent in abrasion resistance and sliding properties at room temperature in particular, which material is used in rotating and sliding sites in office automation apparatuses, electric appliances, computer-related apparatuses, and motors, engines, etc. as automotive parts, etc.
2. Description of the Prior Art
A silicon nitride ceramic is a material, which is lightweight and excellent in heat resistance and abrasion resistance in comparison with metal materials, and is also well balanced between mechanical strength and toughness in comparison with other ceramic materials. Thus, it is expected to be put into practical use in a wide variety of uses for structural parts including engine parts of a car, sliding parts of office automation apparatuses, and heat-resistant and abrasion-resistant parts of a gas turbine engine.
In the prior research and development of a silicon nitride ceramic material for such a variety of uses, the mainstream of material development is mainly aimed at improvements in brittleness and abrasion resistance inherent in the material as well as an improvement in mechanical strengths at high temperatures in the direction of evolution toward high-temperature uses.
For example, Japanese Patent Laid-Open No. 7-267738(1995) discloses an abrasion-resistant silicon nitride ceramic material densified to a theoretical density by twice sintering a powder mixture of Si.sub.3 N.sub.4 admixed with a small amount of a sintering aid composed of MgO, ZrO.sub.2 and CeO.sub.2 in a nitrogen atmosphere containing MgO gas and CO gas to control the amount of the grain boundary phase. Japanese Patent Laid-Open No. 5-155662(1993) discloses a silicon nitride ceramic material likewise densified close to the theoretical density and hence improved in abrasion resistance by primary sintering under ordinary pressure and subsequent secondary sintering in a pressurized atmosphere while controlling the amount of the grain boundary phase of an Al compound, etc. to small one. On the other hand, Japanese Patent Laid-Open No. 63-55163(1988) discloses a silicon nitride ceramic material improved in chipping resistance during sliding by controlling the pore size while providing a porosity of at most 3%.
Further, Japanese Patent Laid-Open No. 6-200936(1994) discloses a high-strength silicon nitride ceramic high-speed bearing material comprising equiaxed .alpha.-grains and needle-like .beta.-grains, and having a dense and fine texture having a grain boundary phase content of at most 15 vol. %, a linear density per 30 .mu.m in length (i.e., the number of grains cut across by a 30 .mu.m line when one section thereof is cut by this line) of at least 35 and a porosity of at most 3%.
Besides, a method wherein a grain boundary glass phase is partly crystallized by carrying out a heat treatment after sintering for forming a dense material has been attempted in order to enhance the mechanical strengths thereof at high temperatures.
On the other hand, in order to improve the sliding surface of a material, very smooth finishing of the surface mainly for mitigation of the initial sliding resistance thereof and formation of a film of a fusion-resistant and agglutination-resistant material on a base material have been actively attempted.
As for the improvements of the bodies of sliding materials themselves in the foregoing description, the bodies are all improved in mechanical strengths and abrasion resistance in such a way as to resist a high load during sliding at ordinary to high temperatures by densification with control of pore formation to the utmost, by fine texturing with control of the amount of a brittle glass phase as the grain boundary phase to the utmost, or partial crystallization of the grain boundary phase.
On the other hand, as for the improvement of the sliding surface, the abrasion resistance and sliding resistance properties thereof are improved by smoothing of the surface or by fresh formation of an alternative material surface.
Meanwhile, uses of sliding parts are not necessarily limited to high-temperature or high-load uses, but include many applications under comparatively low loads in comparison with the above-mentioned uses, at room temperature, for example, uses in rotating or sliding sites of motors in electric appliances, office automation apparatuses, computer-related apparatuses, etc., as well as uses in some automotive parts. In such fields, there is a case where conventional texture designing of a densified ceramic material directed toward higher toughness and strength is not necessarily proper. For example, there is a case where the hardness of a material is particularly increased for an improvement in abrasion resistance at the sacrifice of toughness or with a difficulty in shape fabrication. Thus, much labor is necessary for smoothing a surface by mirror polishing. Further, in order to obtain a dense and high-strength material, many production managements leading to an increase in cost become necessary, which include selection of starting material powders, specification of mixing and molding conditions and sintering conditions, etc. On the other hand, formation of an abrasion-resistant surface portion of an alternative material needs extra labor for such formation, and further always involves problems of deterioration of bonding strength in the formed interface and delamination due to a difference in thermal expansion.
The silicon nitride ceramic material, which is hard to subject to seizing, fusion or agglutination in comparison with metal materials as described above, is expected to be put into practical use as a new sliding material by making the most of its merits, though it involves various problems of production and quality. However, even the foregoing high-performance material is abraded under severe sliding conditions. An abrasion powder worn off is acceleratively formed under some sliding conditions once abrasion begins, whereby the sliding properties may probably be rapidly deteriorated. In an extreme case, sliding surfaces may be mutually locked, thus resulting in no movement thereof. In general, such friction and abrasion phenomena of a ceramic sliding material, though affected by factors in sliding environment, such as sliding pressure, sliding speed, temperature and atmosphere, are notably affected by whether its abrasion resistance on its surface is good or poor. They are of course greatly affected by the surface structure of the material during sliding. As already described, in development of conventional ceramic sliding materials, the main approach to improvements of materials themselves is to cope with sliding environment, and the main approach to improvements of their surface structures is adjustment of initial surface roughness or covering of their surfaces with abrasion-resistant materials.
An improvement in the environmental resistance of a material itself and obtainment of smoothness of a sliding surface are surely fundamental to obtaining a ceramic sliding material excellent in friction and abrasion resistance. In order to suppress the sliding friction and abrasion of a material under circumstances wherein they are satisfied to some extents, nevertheless, how to design the surface structure of an abrasion-resistant material is particularly important. There is a demand for designing a material capable of efficiently exhibiting stable sliding properties in a sliding state with attention focused on the mechanism of the abrasion resistance of the surface of the material during sliding.